Excited states of six oxazine 1 conformers in aqueous solution: TD-DFT/DFT study

•X3LYP/6-31++G(d,p)/SMD theory level gives the best agreement with the experiment.•The shoulder in the experimental spectrum is of vibronic origin.•Differences between conformers are observed for vibronic transitions and relaxation.•With photoexcitation, hydrogen bonds of the dye with water molecules are enhanced. The paper analyzes the electronic-vibrational states of six possible conformers of the dye oxazine 1 (also known as oxazine 725) in an aqueous solution using the SMD solvent model and the 6-31++G(d,p) basis set. Vibronic absorption spectra are calculated using a wide range of hybrid functionals. The X3LYP functional gives the best agreement with the experiment for the positions of the main maximum and shoulder. According to the calculations, the shoulder in the experimental spectrum is of vibronic origin. Various characteristics of the ground and excited states (Duschinsky matrices, IR spectra, atomic charges, dipole moments, maps of the distribution of the electrostatic potential) were obtained. All six investigated conformers have similar vibronic absorption spectra and frontier molecular orbitals. Significant differences between them are observed only for the set of vibronic transitions and relaxation from nonequilibrium to an equilibrium excited state. The effect of two strongly bound water molecules on the excitation of the dye was analyzed.